Sudden deaths from cardiac arrhythmias account for 11% of all natural deaths. Arrhythmias frequently result from treatment with medications, most commonly antiarrhythmic drugs.
Long QT Syndrome (LQTS) is a cardiovascular disorder characterized by prolongation of the QT interval on electrocardiogram and presence of syncope, seizures and sudden death. Five genes have been implicated in Romano-Ward syndrome, the autosomal dominant form of LQTS. These genes are KVLQT1, HERG, SCN5A, KCNE1 and KCNE2. Mutations in KVLQT1 and KCNE1 also cause the Jervell and Lange-Nielsen syndrome, a form of LQTS associated with deafness, a phenotypic abnormality inherited in an autosomal recessive fashion.
The present invention relates to an alteration in the SCN5A gene and methods for detecting the alteration.
The publications and other materials used herein to illuminate the background of the invention or provide additional details respecting the practice, are incorporated by reference, and for convenience are respectively grouped in the appended List of References.
The present invention is directed to an alteration in the SCN5A gene and its gene products which are associated with drug-induced torsade de pointes and ventricular fibrillation and to a process for the diagnosis and prevention of these disorders. These drug-induced physical disorders are diagnosed in accordance with the present invention by analyzing the DNA sequence of the SCN5A gene of an individual to be tested and comparing the DNA sequence to the known DNA sequence of the normal gene. Prediction of drug-induced torsade de pointes or ventricular fibrillation will enable practitioners to prevent this disorder using existing medical therapy.
Cardiac arrhythmias are a common cause of morbidity and mortality, accounting for approximately 11% of all natural deaths (Kannel, 1987; Willich et al., 1987). In general, presymptomatic diagnosis and treatment of individuals with life-threatening ventricular tachyarrhythmias is poor, and in some cases medical management actually increases the risk of arrhythmia and death (Cardiac Arrhythmia Suppression Trial II Investigators, 1992). These factors make early detection of individuals at risk for cardiac arrhythmias and arrhythmia prevention high priorities.
Both genetic and acquired factors contribute to the risk of developing cardiac arrhythmias. Long QT syndrome (LQT) is an inherited cardiac arrhythmia that causes abrupt loss of consciousness, syncope, seizures and sudden death from ventricular tachyarrhythmias, specifically torsade depointes and ventricular fibrillation (Ward, 1964; Romano, 1965; Schwartz et al., 1975; Moss et al., 1991). This disorder usually occurs in young, otherwise healthy individuals (Ward, 1964; Romano, 1965; Schwartz et al., 1975). Most LQT gene carriers manifest prolongation of the QT interval on electrocardiograms, a sign of abnormal cardiac repolarization (Vincent et al., 1992). The clinical features of LQT result from episodic cardiac arrhythmias, specifically repolarization-related ventricular tachyarrhythmias like torsade de pointes, named for the characteristic undulating nature of the electrocardiogram in this arrhythmia and ventricular fibrillation (Schwartz et al., 1975; Moss and McDonald, 1971). Torsade de pointes may degenerate into ventricular fibrillation, a particularly lethal arrhythmia. Although LQT is not a common diagnosis, ventricular arrhythmias are very common; more than 300,000 United States citizens die suddenly every year (Kannel, et al., 1987; Willich et al., 1987) and, in many cases, the underlying mechanism may be aberrant cardiac repolarization. LQT, therefore, provides a unique opportunity to study life-threatening cardiac arrhythmias at the molecular level.
Both inherited and acquired forms of LQT have been defined. Acquired LQT and secondary arrhythmias can result from cardiac ischemia, bradycardia and metabolic abnormalities such as low serum potassium or calcium concentration (Zipes, 1987). LQT can also result from treatment with certain medications, including antibiotics, antihistamines, general anesthetics, and, most commonly, antiarrhythmic medications (Zipes, 1987). Inherited forms of LQT can result from mutations in at least five different genes. In previous studies, LQT loci were mapped to chromosome 11p15.5 (KVLQT1 or LQT1) (Keating et al., 1991a; Keating et al., 1991b), 7q35–36 (HERG or LQT2), 3p21–24 (SCN5A or LQT3) (Jiang et al., 1994). Of these, the most common cause of inherited LQT is KVLQT1. Our data indicate that mutations in this gene are responsible for more than 50% of inherited LQT. A fourth LQT locus (LQT4) was mapped to 4q25–27 (Schott et al., 1995). Also, KCNE1 (LQT5) has been associated with long QT syndrome (Splawski et al., 1997b; Duggal et al., 1998). These genes encode ion channels involved in generation of the cardiac action potential. Mutations can lead to channel dysfunction and delayed myocellular repolarization. Because of regional heterogeneity of channel expression with the myocardium, the aberrant cardiac repolarization creates a substrate for arrhythmia. KVLQT1 and KCNE1 are also expressed in the inner ear (Neyroud et al., 1997; Vetter et al., 1996). We and others demonstrated that homozygous or compound heterozygous mutations in each of these genes can cause deafness and the severe cardiac phenotype of the Jervell and Lange-Nielsen syndrome (Neyroud et al., 1997; Splawski et al., 1997a; Schultze-Bahr et al., 1997; Tyson et al., 1997). Loss of functional channels in the ear apparently disrupts the production of endolymph, leading to deafness.
Presymptomatic diagnosis of LQT is currently based on prolongation of the QT interval on electrocardiograms. QTc (QT interval corrected for heart rate; Bazzett, 1920) greater than 0.44 second has traditionally classified an individual as affected. Most LQT patients, however, are young, otherwise healthy individuals, who do not have electrocardiograms. Moreover, genetic studies have shown that QTc is neither sensitive nor specific (Vincent et al., 1992). The spectrum of QTc intervals for gene carriers and non-carriers overlaps, leading to misclassifications. Non-carriers can have prolonged QTc intervals and be diagnosed as affected. Conversely, some LQT gene carriers have QTc intervals of ≦0.44 second but are still at increased risk for arrhythmia. Correct presymptomatic diagnosis is important for effective, gene-specific treatment of LQT.
Autosomal dominant and autosomal recessive forms of this disorder have been reported. Autosomal recessive LQT (also known as Jervell and Lange-Nielsen syndrome) has been associated with congenital neural deafness; this form of LQT is rare (Jervell and Lange-Nielsen, 1957). Autosomal dominant LQT (Romano-Ward syndrome) is more common, and is not associated with other phenotypic abnormalities (Romano et al., 1963; Ward, 1964). A disorder very similar to inherited LQT can also be acquired, usually as a result of pharmacologic therapy (Schwartz et al., 1975; Zipes, 1987).
The data have implications for the mechanism of arrhythmias in LQT. Two hypotheses for LQT have previously been proposed (Schwartz et al., 1994). One suggests that a predominance of left autonomic innervation causes abnormal cardiac repolarization and arrhythmias. This hypothesis is supported by the finding that arrhythmias can be induced in dogs by removal of the right stellate ganglion. In addition, anecdotal evidence suggests that some LQT patients are effectively treated by β-adrenergic blocking agents and by left stellate ganglionectomy (Schwartz et al., 1994). The second hypothesis for LQT-related arrhythmias suggests that mutations in cardiac-specific ion channel genes, or genes that modulate cardiac ion channels, cause delayed myocellular repolarization. Delayed myocellular repolarization could promote reactivation of L-type calcium channels, resulting in secondary depolarizations (January and Riddle, 1989). These secondary depolarizations are the likely cellular mechanism of torsade de pointes arrhythmias (Surawicz, 1989). This hypothesis is supported by the observation that pharmacologic block of potassium channels can induce QT prolongation and repolarization-related arrhythmias in humans and animal models (Antzelevitch and Sicouri, 1994). The discovery that one form of LQT results from mutations in a cardiac potassium channel gene supports the myocellular hypothesis.
In theory, mutations in a cardiac sodium channel gene could cause LQT. Voltage-gated sodium channels mediate rapid depolarization in ventricular myocytes, and also conduct a small current during the plateau phase of the action potential (Attwell et al., 1979). Subtle abnormalities of sodium channel function (e.g., delayed sodium channel inactivation or altered voltage-dependence of channel inactivation) could delay cardiac repolarization, leading to QT prolongation and arrhythmias. In 1992, Gellens and colleagues cloned and characterized a cardiac sodium channel gene, SCN5A (Gellens et al., 1992). The structure of this gene was similar to other, previously characterized sodium channels, encoding a large protein of 2016 amino acids. These channel proteins contain four homologous domains (DI-DIV), each of which contains six putative membrane spanning segments (S1–S6). SCN5A was mapped to chromosome 3p21, making it an excellent candidate gene for LQT3 (George et al., 1995), and this gene was then proved to be associated with LQT3 (Wang et al., 1995).
In 1994, Warmke and Ganetzky identified a novel human cDNA, human ether a-go-go related gene (HERG, Warmke and Ganetzky, 1994). HERG was localized to human chromosome 7 by PCR analysis of a somatic cell hybrid panel (Warmke and Ganetzky, 1994) making it a candidate for LQT2. It has predicted amino acid sequence homology to potassium channels. HERG was isolated from a hippocampal cDNA library by homology to the Drosophila ether a-go-go gene (eag), which encodes a calcium-modulated potassium channel (Bruggemann et al., 1993). HERG is not the human homolog of eag, however, sharing only ˜50% amino acid sequence homology. HERG has been shown to be associated with LQT2 (Curran et al., 1995).
LQT1 was found to be linked with the gene KVLQT1 (Q. Wang et al., 1996). Sixteen families with mutations in KVLQT1 were identified and characterized and it was shown that in all sixteen families there was complete linkage between LQT1 and KVLQT1. KVLQT1 was mapped to chromosome 11p15.5 making it a candidate gene for LQT1. KVLQT1 encodes a protein with structural characteristics of potassium channels, and expression of the gene as measured by Northern blot analysis demonstrated that KVLQT1 is most strongly expressed in the heart. One intragenic deletion and ten different missense mutations which cause LQT were identified in KVLQT1. These data define KVLQT1 as a novel cardiac potassium channel gene and show that mutations in this gene cause susceptibility to ventricular tachyarrhythmias and sudden death.
It was known that one component of the IKs channel is minK, a 130 amino acid protein with a single putative transmembrane domain (Takumi et al., 1988; Goldstein and Miller, 1991; Hausdorff et al., 1991; Takumi et al., 1991; Busch et al., 1992; Wang and Goldstein, 1995; KW Wang et al., 1996). The size and structure of this protein made it unlikely that minK alone forms functional channels (Attali et al., 1993; Lesage et al., 1993). Evidence was presented that KVLQT1 and minK coassemble to form the cardiac IKs potassium channel (Sanguinetti et al., 1996). IKs dysfunction is a cause of cardiac arrhythmia. It was later shown that mutations in KCNE1 (which encodes minK) also can result in LQT (Splawski et al., 1997b).